Why does salt melt ice, and what else is it doing?

This is a common scene in the Northern states and Canada, the salt truck spreading rock salt on the roads. According to Slate, more than 20 million tons of the stuff is spread each year, 13 times more than is used by the entire food processing industry. According to the USGS,

Salt is an inexpensive, widely available, and effective ice control agent. It does, however, become less effective as the temperature decreases below about -6.5° C to -9.5° C (15° F to 20° F). at lower temperatures, more salt would have to be applied to maintain higher brine concentrations to provide the same degree of melting. Most winter snowstorms and ice storms happen when temperatures are between -4° C and 0° C (25° F and 32° F), the range in which salt is most effective.

Salt works by lowering the freezing point of water. When sprinkled on ice, it makes a brine with the film of surface water, which lowers the freezing point and starts melting the ice that the brine is in contact with- to a point. The lower the temperature, the more salt you need, so it is less useful below -10C (15F). That's why in a lot of really cold places they use sand on top of the snow, and why places like Quebec make snow tires mandatory- they spend a lot of time driving on top of snow instead of road.

The environmental costs are huge

The problem with salt is that it has nowhere to go but down, into the groundwater and then into rivers and streams. A study in Pickering, Ontario (just east of Toronto) found that the salt was flowing into Frenchman's Bay, where it is affecting the fish population. According to the Globe and Mail,

Environment Canada has recognized that salt has adverse impacts on wildlife, plants, water and soil, and in 2001 considered adding it to the country's list of the most toxic substances...."It's a toxic material and yet we continue to throw it with gay abandon on our roads."

However, the enormous hidden cost is not immediately seen, but is added to the deferred maintenance problems which will be paid in future budgets. Over the next 10 years, Michigan will theoretically spend $5 billion on road salt and its correlated depreciation to infrastructure investment.

According to Environment Canada, road salt causes $ 143 in depreciation every year for every car on the salty road.

What are the alternatives?

Road salt destroys roads, shortens the lives of cars, kills vegetation and now, we know that it is harming our watersheds. Better alternatives would be to reduce speed limits in winter, make snow tires mandatory as they do in Quebec, and provide better public transit and other alternatives to driving, instead of destroying the environment to satisfy a need for speed.